Power supplies typically require some form of current-limiting so as to prevent excess currents from damaging components of the power supply during an output short circuit condition. In many power converter topologies, such as those referred to as flyback (discontinuous current), forward (continuous current), and bridge configurations, a switching device, such as a transistor, is referenced to a ground potential. It is consequently relatively straight forward to sense the output current with a resistor connected between the switching transistor and ground. A voltage developed across this resistor by the current can be used to shut off the transistor during an over-current condition.
Examples of circuits that sense a current in a source of a ground referenced switching transistor are found in U.S. Pat. Nos. 4,811,184 and 4,672,518.
However, of particular interest herein is a DC to DC converter known in the art as a buck converter.
FIG. 1 shows an example of a conventional buck converter, specifically a single quadrant-type converter. A characteristic of the buck converter is that the load voltage is equal to or less than the source voltage. A switching, or buck, transistor is connected between an unregulated DC source and an inductance (L). The inductance L functions as a current source and, in conjunction with a capacitor C, forms an LC filter that limits ripple voltage at the load. An exemplary control network includes an operational amplifier configured as a voltage comparator that generates, in conjunction with resistor R and voltage reference (V.sub.REF), a variable duty cycle switching signal to the base of the switching transistor. It should be noted that the illustrated bipolar buck transistor could be replaced with, by example, a field effect transistor (FET) or with a gate turn-off thyristor. The free wheeling diode functions as an autocomplementary switch. That is, the free wheeling diode turns on automatically to supply the demands of the current source whenever the switching transistor is turned off, and turns off as a result of commutations driven by the source voltage whenever the switching transistor turns on.
For the buck converter of FIG. 1, the output DC voltage is given by: EQU Output DC voltage=D.times.input DC voltage,
where D is the duty cycle of the switching transistor.
As can be appreciated, for the buck converter the above mentioned technique of sensing the output current with a resistor connected between the switching transistor and ground cannot be used, in that both the input and output of the switching transistor are referenced to the power bus, and not to ground potential.
It is also not possible to use the transformed inductor current, in that this current contains no DC current information.
One technique for generating an over-current indication places a resistor in series with the power bus, and then employs a differential amplifier to remove the common-mode bus voltage. However, this may be difficult to accomplish if the voltage developed across the resistor (normal-mode) is small compared to the common-mode voltage. Also, it may be difficult to provide sufficient bandwidth in the differential amplifier to generate the ground-referenced information rapidly enough to protect the circuitry from an over-current condition.
Another possible technique, and the one most commonly used, is to place a resistor in the power return path and to use the voltage induced across the resistor to sense fault currents. However, this technique generates a voltage between the power return and the output return, which may be objectionable for safety reasons. This technique may also give rise to Electromagnetic Interference (EMI) problems. A further problem is that the ground current is DC, since it is smoothed by the power supply filter, and consequently cannot be relied on to change rapidly. A still further problem results from the fact that the power elements in the buck converter are in the high-side (power bus) current path. Consequently, in the case of a ground fault, these elements may suffer excessive stress without the current sensor detecting the stress, because the excess current is shunted around the resistor by the ground fault.
In U.S. Pat. No. 4,672,303 there is described, in FIG. 8, a buck-like converter that includes a flywheel FET 32 that is connected to a control circuit so as to be turned off at a predetermined minimum level of inductor current. Current limiting circuitry is provided to insure that the current in the flywheel FET, and in the inductor, has reversed before the the gate drive is removed from the flywheel FET. The current limit circuit includes a FET 81 that is connected in parallel with the flywheel FET, and a resistor that is connected in series with the FET 81. A comparator 82 has one input referenced to ground and a second input connected between the FET 81 and the series resistor. When the flywheel FET is conducting, the parallel FET 81 is saturated, and provides a low impedance path for the voltage across the flywheel FET to the input of the comparator. The series resistance has a relatively high value. In a normal mode of operation, the control circuit is said to determine when the flywheel FET is turned off. During a current-limiting mode of operation, the comparator 82 controls the timing of the turn-off of the flywheel FET. As such, this patent does not address the sensing of a current that flows in the high-side buck transistor (31).
Another type of power conversion circuit is known as a bilateral converter. This type of converter is capable of operating in either a buck configuration or in a boost configuration. Over-current sensing in this type of converter is complicated by the fact that current is capable of flowing in two directions through circuit components, depending on whether the bilateral converter is being operated in a buck or a boost mode.
An object of this invention is to provide a bilateral power converter that employs an over-current sensing element capable of sensing current flowing in either direction through switching devices.
A further object of this invention to provide a current-limited buck converter that overcomes the aforementioned problems inherent in conventional buck converter current-limiting approaches.
A still further object of the invention is to overcome the disadvantages inherent in conventional buck converter current-limiting approaches by sensing the current in the free-wheeling diode in such a manner that an indication is obtained of a current flowing in the power bus, including the buck transistor.